Structural arrangements for the open-circuit liquid cooling of gas turbine buckets are shown by Kydd, U.S. Pat. Nos. 3,445,481 and 3,446,482. The first patent discloses a bucket having cooling passages open at both ends which are defined by a series of ribs forming part of the core portion of the bucket and a sheet metal skin covering the core and welded to the ribs. The second patent discloses squirting liquid under pressure into hollow forged or cast turbine buckets. Another patent issued to Kydd, U.S. Pat. No. 3,619,076 describes an open circuit cooling system wherein a turbine blade construction consists of a central airfoil-shaped spar which is clad with a sheet of metal having a very high thermal conductivity, e.g. copper. The cladding sheet has grooves recessed in the sheet face adjacent to the spar, which grooves together with the smooth surface of the spar to define coolant passages distributed over the surface of the turbine blade.
There are some disadvantages in forming liquid cooling passages by bonding a sheet to a core in either of the configurations shown in U.S. Pat. Nos. 3,445,481 or 3,619,076. Thus, when a braze is used to bond the skin, some channels of the turbine buckets become plugged and obstructed with braze material. Excellent bonds are required between the core and the skin to contain the water in full channel flow under extremely high hydraulic pressures which result from the centrifugal forces during operation of the turbine. In addition, any cracks in the skin can cause leakage of the coolant and result in vane failure.
Many of the disadvantages of the prior art are overcome by the invention disclosed in the copending application of Anderson, "Liquid Cooled Gas Turbine Buckets", Ser. No. 749,719, filed on Dec. 13, 1976, of common assignee. Anderson discloses water cooled turbine buckets wherein the water-cooling channels are formed using preformed tubes which are located beneath an outer protective layer composed of an inner skin to provide high thermal conductivity and an outer skin to provide protection from hot corrosion.
Also Schilling, et al., U.S. Pat. No. 3,928,901 and Schilling, et al., U.S. Pat. No. 3,952,939 both disclose methods of attaching sheet cladding to a convex-concave substrate such as an air foil or a turbine bucket using isostatic pressing techniques. However, the procedures set forth in these Schilling patents when applied to the manufacture of turbine buckets incorporating preformed tubes will tend to collapse the tubes. Furhermore, when molten glass is used as the pressure transmitting medium as disclosed in U.S. Pat. No. 3,952,939, the molten glass is able to enter the tubes and is then difficult or almost impossible to remove without damage to the tubes.